linux-kernel - Re: [PATCH] PM: hibernate: clean up sync_read handling in snapshot_write

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Date:   Tue, 26 Sep 2023 16:02:34 -0400
From:   Brian Geffon <bgeffon@...gle.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Matthias Kaehlcke <mka@...omium.org>, stable@...r.kernel.org
Subject: Re: [PATCH] PM: hibernate: clean up sync_read handling in snapshot_write_next

On Tue, Sep 26, 2023 at 2:24 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Fri, Sep 22, 2023 at 6:07 PM Brian Geffon <bgeffon@...gle.com> wrote:
> >
> > In snapshot_write_next sync_read is set and unset in three different
> > spots unnecessiarly. As a result there is a subtle bug where the first
> > page after the meta data has been loaded unconditionally sets sync_read
> > to 0. If this first pfn was actually a highmem page then the returned
> > buffer will be the global "buffer," and the page needs to be loaded
> > synchronously.
> >
> > That is, I'm not sure we can always assume the following to be safe:
> >                 handle->buffer = get_buffer(&orig_bm, &ca);
> >                 handle->sync_read = 0;
> >
> > Because get_buffer can call get_highmem_page_buffer which can
> > return 'buffer'
> >
> > The easiest way to address this is just set sync_read before
> > snapshot_write_next returns if handle->buffer == buffer.
> >
> > Signed-off-by: Brian Geffon <bgeffon@...gle.com>
> > Fixes: 8357376d3df2 ("[PATCH] swsusp: Improve handling of highmem")
> > Cc: stable@...r.kernel.org
>
> If you send an update of a patch, it is always better to give it a
> higher version number to avoid any possible confusion.

Yes, I apologize. I wanted to add a Fixes line and I wasn't sure what
the best practice was. I'll do my best to not create any additional
work for you going forward!

>
> > ---
> >  kernel/power/snapshot.c | 6 +-----
> >  1 file changed, 1 insertion(+), 5 deletions(-)
> >
> > diff --git a/kernel/power/snapshot.c b/kernel/power/snapshot.c
> > index 190ed707ddcc..362e6bae5891 100644
> > --- a/kernel/power/snapshot.c
> > +++ b/kernel/power/snapshot.c
> > @@ -2780,8 +2780,6 @@ int snapshot_write_next(struct snapshot_handle *handle)
> >         if (handle->cur > 1 && handle->cur > nr_meta_pages + nr_copy_pages + nr_zero_pages)
> >                 return 0;
> >
> > -       handle->sync_read = 1;
> > -
> >         if (!handle->cur) {
> >                 if (!buffer)
> >                         /* This makes the buffer be freed by swsusp_free() */
> > @@ -2824,7 +2822,6 @@ int snapshot_write_next(struct snapshot_handle *handle)
> >                         memory_bm_position_reset(&zero_bm);
> >                         restore_pblist = NULL;
> >                         handle->buffer = get_buffer(&orig_bm, &ca);
> > -                       handle->sync_read = 0;
> >                         if (IS_ERR(handle->buffer))
> >                                 return PTR_ERR(handle->buffer);
> >                 }
> > @@ -2834,9 +2831,8 @@ int snapshot_write_next(struct snapshot_handle *handle)
> >                 handle->buffer = get_buffer(&orig_bm, &ca);
> >                 if (IS_ERR(handle->buffer))
> >                         return PTR_ERR(handle->buffer);
> > -               if (handle->buffer != buffer)
> > -                       handle->sync_read = 0;
> >         }
> > +       handle->sync_read = (handle->buffer == buffer);
> >         handle->cur++;
> >
> >         /* Zero pages were not included in the image, memset it and move on. */
> > --
>
> Anyway, applied as 6.7 material with some minor edits in the subject
> and changelog.

Thank you for taking the time to review!
Cheers

>
> Thanks!